This invention relates to the use of arsenic selenium photoconductive layers in electrophotography, and, in particular, to an apparatus and method of making multiple copies of an original document when using arsenic selenide as the photoconductor without observing objectionable image density variations between the various copies produced.
Electrophtographic copy machines are known which are based on the development of an electrostatic image presented by a photoconductive element and perform the process steps which include placing a uniform electrostatic charge on the photoconductor, exposing the charged photoconductor to a light image obtained from the document to be copied thereby creating on the surface of the photoconductor a differential potential pattern in accordance with the light image, developing the differential potential pattern by the presentation of toner particles to the photoconductor using a development technique capable of providing uniformly-filled solid areas, transferring the toner developed image directly or indirectly to a receptor, such as paper, fixing the toner to the paper and a cleaning step for removing residual toner on the photoconductor following exposure of the photoconductor to a pre-clean light source.
When photoconductive elements using a layer formed of glasses of the arsenic selenium system as disclosed in U.S. Pat. Nos. 2,803,542 to Ullrich and 2,822,300 to Mayer are used as the photoconductor in a copy machine employing the copy process described above and a number of copies of an original document are made in rapid succession, a noticeable and objectionable decrease in image density will occur between the first and second copy with additional image density degradations or variations occurring to some degree with each successive copy made.
These observable copy density variations are the result of the well-known light fatigue effect exhibited by many materials containing selenium and particularly by arsenic selenide wherein an increase in the rate of dark decay of the surface potential has been observed with repeated cycles of charging and exposure. The amount of fatigue or the fatigue level will eventually reach a maximum after a period of continuous cycling.
Generally, light fatigue can be measured by monitoring the surface potential response of a photoconductor in the development region as the photoconductor completes repeated cycles while functioning to reproduce a particular document containing both solid black and gray areas. A cyclic decrease in the measurable surface potential is caused by light fatigue and will result in a degradation in image density with successive copies when a development technique is used that is sensitive to absolute potential levels on the photoconductor. Such development techniques are generally capable of providing uniformly-filled solid image areas.
One solution to the fatigue problem and, therefore, the variable copy density problem presented by a photoconductor of the arsenic selenium system is disclosed in U.S. Pat. No. 3,511,649 to E. J. Felty et al which recognizes that the fatigue of photoconductors of the arsenic selenium system is wavelength dependent. The patent to Felty et al provides for reduction of the fatigue to an acceptable level by using interference filters to cut out all wavelengths from the imaging light source that exceed about 5400 angstroms or 540 nanometers.